Band rejection filters are required for suppressing undesirable frequency bands. Such filters can advantageously be used, for example, if, in the case of two frequency bands that are close together in a wireless transmission standard, one of the two bands is intended to be masked out. Ideally, the passband of the filter should have at least the bandwidth of the system to be received and should generate only a low insertion loss there. In the stop band, such a filter should have a high attenuation and ideally have a fast transition from passband to stop band, that is to say a steep edge of the passband.
Filters based on combinations of coils and capacitors are known, which, however, require an extremely high quality factor of the elements and high filter degrees in order to satisfy the requirements provided in respect of bandwidth, insertion loss and rejection performance. Moreover, it is virtually impossible to realize such filters cost-effectively and in a small fashion.
Band rejection filters are known, for example from the documents EP 2093881 A1, WO 2010/001522, EP 1976116 A1, EP 1986320 A1 and EP 1944867 A1.
Furthermore, SAW filters comprising resonators as a band rejection filter are known. By way of example, the document U.S. Pat. No. 4,803,449 describes a ladder arrangement comprising SAW resonators in series branches or parallel branches. Parallel resonators comprising series inductances are known, for example, in the document EP 1398876 A1.